1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to golf balls including a core and a cover.
2. Description of the Related Art
A general golf ball includes a core and a cover. For the cover, a resin composition including an ionomer resin or polyurethane as a principal component is used. A cover including ionomer resin has excellent resilience performance. On the other hand, a cover including polyurethane has excellent feel at impact, excellent controllability, and excellent scuff resistance.
Examples of methods for forming a cover include compression molding and injection molding. In compression molding, a core is covered with bowl-shaped half shells that are prepared in advance, and then compression-molded to form a cover. In injection molding, a melt resin is injected directly around a core with an injection molding machine, to form a cover. In light of productivity, injection molding is superior to compression molding.
JPH11-151323 (U.S. Pat. No. 6,190,597) discloses a golf ball including a thin spherical layer (corresponding to a cover) that is formed with uniform thickness by injection molding. The injection rate is optimized in injection molding, and the thin spherical layer is formed.
JP No. 3751824 (U.S. Pat. No. 6,235,230) discloses a mold for use in injection molding for forming a cover. The mold has a ring gate. The ring gate achieves balanced flow of a melt resin, thereby improving the durability of a golf ball.
In light of achieving a long flight distance, increasing the diameter of a core in order to cause a golf ball to fly at a high speed is studied. For increasing the diameter of the core, a cover needs to be thinner.
If an injection rate of an injection molding machine is insufficient when forming a thin cover, the cooling speed of an injected resin composition increases, and hence the resin composition may not be able to entirely cover the core. Even if the core is entirely covered with the resin composition, the hit durability of a golf ball may deteriorate because the strength of the golf ball at the weld line is insufficient. When only molding conditions such as temperature and pressure are controlled, a cover with sufficient characteristics cannot be obtained, and the life of the mold is also shortened.
When a flow improver such as oil is added to a resin composition for improving flow of the resin composition, there is a problem that characteristics such as resilience performance, durability, and appearance deteriorate. In injection molding that provides excellent productivity, it is difficult to achieve a thin cover without impairing the characteristics of the cover.
An objective of the present invention is to provide a golf ball that is manufactured by injection molding, has excellent durability, and achieves a long flight distance.